Sillimanite (Al₂SiO₅) is the high-temperature aluminosilicate polymorph (with kyanite high-P, andalusite low-P/T). It forms long fibrous prismatic crystals in high-grade metamorphic rocks. The fibrous variety “fibrolite” is occasionally cut as gemstone for cat’s-eye effect. Sri Lanka and Madagascar supply gem material.
Key Facts
- Mohs hardness 6.5–7.5.
- Orthorhombic; one of three Al₂SiO₅ polymorphs.
- Long fibrous prismatic crystal habit.
- Diagnostic of upper amphibolite to granulite facies metamorphism.
- Industrial source of mullite refractories for high-T applications.
Notable Localities
Sri Lanka and Madagascar yield gem fibrolite. Mogok (Myanmar) supplies cat’s-eye material. Brandywine (Delaware) is the type-locality.
Found at these Localities
- Mogok Stone Tract (抹谷宝石产地)
